1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a porous silicon photo-device using porous silicon.
2. Description of the Related Art
Semiconductors such as Si and Ge that are group IV elements of the periodic table are essentially indirect transition semiconductors and hence have been conventionally thought of as capable of only nonluminescent transition. However, in recent years it has turned out that even these semiconductors show luminescent transition characteristics when they are made porous and given a micro-structure by means of, e.g., anodization. EL (ElectroLuminescence) devices fabricated by using these indirect transition semiconductors having a micro-structure are attracting a great deal of attention, since these devices luminesce at low luminescence threshold voltages. Therefore, research and development of these EL devices are being extensively done.
By convention, the basic arrangement of the EL devices as discussed above consists of a semiconductor device fabricated by stacking a luminescing semiconductor layer having a micro-structure (porous structure) and a conductive layer into which an electric charge is injected. The conductive layer is made from Au, ITO, or SiC and stacked on a layer of Si or Ge by using a thin film formation technique such as vapor deposition or sputtering. It is already confirmed that these devices luminesce, albeit very weakly, upon application of a voltage of several volts or higher. Also, some researchers are pursuing a method in which a p-n junction is formed in a semiconductor layer and made porous to have a micro-structure, thereby making both electrons and holes simultaneously present in this micro-structure. In this method luminescence is caused by recombination of these electrons and holes in the micro-structure.
The light emission efficiency of these EL devices is low compared to that of PL (PhotoLuminescence) which is obtained by directly irradiating exciting light. The stability of the luminescence of the EL device is also unsatisfactory. Additionally, the electrical conductivity of the EL device is low since the electrical resistance of the device itself is high. To use the EL devices as photoelectric conversion devices in the future, therefore, this low electrical conductivity will be troublesome because these devices are supposed to handle high-speed signals. One cause of these problems is the lack of mechanical strength of the device. That is, in order to increase the light emission efficiency, it is necessary to increase the porosity of a portion of a semiconductor which is given a micro-structure. Accordingly, the mechanical strength of this micro-structure decreases significantly. As a consequence, cracks are often formed on the surface of the device in operation, or the device causes uneven luminescence in many instances. In some cases the semiconductor device is used as another device such as a rectifier device. In these instances the low mechanical strength results in unstable current-voltage characteristics of the device, if a charge injection layer is formed by vapor deposition or sputtering.
A cause of these drawbacks of the porous silicon device is considered as follows. That is, when the surface of a silicon substrate is made porous by, e.g., anodization, to form a silicon micro-skeleton consisting of a number of silicon wires, and when the resultant surface is exposed to the air, a natural oxide film is formed on the silicon wires and at the same time the silicon wires deform and aggregate. This decreases the mechanical strength of the micro-skeleton and also makes the charge injection layer formed on it nonuniform. Consequently, the light emission efficiency decreases, and the stability of luminescence becomes unsatisfactory. The electrical conductivity also lowers.
As discussed above, the basic form of the light-emitting device using porous silicon having a micro-skeleton as a base is the structure in which a light-transmitting charge injection layer is stacked on the porous silicon. That is, the device is a heterojunction device fabricated by stacking an n-type semiconductor, such as ITO (Indium Tin Oxide), on a p-type porous layer. Examples of this heterojunction device are a device in which the electrode is constructed of a semi-transparent metal, and a device in which a polymer is formed by electrolytic polymerization. It is unfortunate that neither of these devices has a sufficient light emission efficiency; the light emission efficiency is at most 10.sup.-6 to 10.sup.-4 %. One possible method by which this is improved is to form a homo p-n junction of the porous silicon. Several attempts have been made to achieve this method. As an example, in Fraunhofer-Institute in Germany a p-type layer was formed on the surface of n-type silicon and a p-n junction was formed by irradiation of light (W. Lang et al., "Porous silicon light-emitting p-n junction", Journal of Luminescence 57 (1993), pp. 169-173). Unfortunately, by this method the surface is a p-type layer and this makes it impossible to combine the device with an ITO film which is effective as a light-transmitting charge injection layer. This is because ITO is of n-type and hence forms a p-n junction with the surface of the p-type layer, with the result that an n-p-n junction is formed by this p-n junction and the internal junction. IBM Corp. has reported a method in which a p-n junction is formed by forming an n-type silicon layer on a p-type silicon layer and is selectively etched to form a number of mesa regions in which the p-n junction surface is exposed, and the underlying p-type layer is selectively made porous to form a pseudo p-n junction (E. Bassous et al., "Characterization of Microporous Silicon fabricated by Immersion Scanning", Mat. Res. Soc. Symp. Proc. Vol. 256, pp. 23-26). In this method, however, the resultant p-n junction is essentially a heterojunction since the overlying n-type layer is made from bulk silicon, so only a low light emission efficiency can be attained.
To solve these problems, it is only necessary to obtain a homo junction having an n-type porous layer on the surface and a p-type porous layer as the underlying layer. Unfortunately, it is very difficult to form this structure such that the structure is uniform and both the n-type and the p-type layers are suitable for luminescence. More specifically, it is assumed possible to form a junction of porous silicon while well controlling the depth and the impurity concentration, by forming an n-type layer on the surface of p-type bulk silicon by impurity diffusion and anodizing the resultant structure from the surface. However, by this method the anodization does not uniformly proceed, i.e., the anodization is concentrated in a portion of the anodized region. The result is a nonuniform junction structure in which the p-type porous layer is formed in a portion below the n-type porous silicon layer. The reason for this is considered that since the anodization reaction originally requires holes, the p-type layer having a large number of holes is readily anodized significantly, and consequently the anodization selectively proceeds in a portion where the anodization has first reached the underlying p-type layer. For this reason, the resultant junction luminesces only nonuniformly, so it is not possible to obtain satisfactory performance.
The heterojunction device is also considered as a Schottky junction between porous silicon and, e.g., ITO or Au. Therefore, no large current flows unless the thermoelectronic emission is increased. Consequently the bias voltage needs to be set higher than the Schottky barrier. If the current is increased, an energy loss caused by a high series resistance increases, and this makes high-efficiency luminescence difficult.
In addition, when a p-n junction of a porous semiconductor is formed by making a p-n junction substrate porous, the morphological structure of the porous silicon in the p-type region differs from that in the n-type region. Consequently, the band gap in the p-type layer is also different from that in the n-type layer. For that reason, only p-n hetero-junctions have been realized to date. In particular, it is difficult to fabricate a micro-structure which emits visible light by using heavily doped silicon as a p-type layer. The resistance of porous silicon fabricated on a low-resistance substrate is further increased when the silicon is made porous. As discussed above, p-n junction devices using conventional porous semiconductors are actually p-n heterojunction devices in which the resistance of the light-emitting layer is high. Therefore, no p-n junction devices having satisfactory electrical characteristics and luminescence characteristics have been obtained yet.
Incidentally, various types of optical semiconductor elements (to be referred to as photoelectric conversion elements hereinafter) which receive light and output a current or a voltage have been developed and put into practical use. In devices such as photocouplers requiring integration, photoelectric conversion elements (silicon photoelectric conversion elements) such as photodiodes or phototransistors using silicon, among other photoelectric conversion elements, have been extensively used because of good matching properties with an output processing circuit or with some other peripheral device and a high quantum efficiency. Various structures are known as the structure of the silicon photoelectric conversion element. Examples are a p-n junction, a p-i-n junction, and a metal silicon Schottky junction. The light reception characteristics and the junction depth of these photoelectric conversion elements are controlled by adjusting the impurity concentration. Consequently, photoelectric conversion elements having different light reception wavelength sensitivity peaks are being fabricated.
Usually, the region of the light reception sensitivity of the silicon photoelectric conversion element is in the region of a wavelength longer than 0.6 .mu.m. The sensitivity of devices most widely used as the light-receiving element is in a wavelength region of approximately 0.9 .mu.m (regions from the visible region to the infrared region). Therefore, silicon photoelectric conversion elements usable as the light-receiving element of a photocoupler are restricted to those having a luminescence wavelength in regions from the visible region to the infrared region. For this reason, Si-doped compounds such as GaAs, GaAsP, and GaAlAs are used as the material of the light-emitting element. These materials are entirely different from the materials of the silicon photoelectric conversion elements. Therefore, in the manufacture of a photocoupler the light-receiving element and the light-emitting element are formed independently. The light-receiving elements and the light-emitting elements thus fabricated are mounted on lead frames or the like, and these lead frames are so assembled as to oppose each other in a so-called coupling step. Subsequently, a transparent silicone resin or the like material is filled between these light-emitting and light-receiving elements, and the coupled elements are transfer-molded with a white epoxy resin.
It is unfortunate that conventional photocouplers as discussed above have the problem of a high manufacturing cost resulting from a cumbersome coupling step. In addition, this coupling step is the major cause of variations in the optical coupling efficiency between the light-receiving and light-emitting elements. This is so because it is unavoidable that the positions slightly deviate in each of the steps of die-bonding the elements to the lead frames, assembling the lead frames to oppose each other, and bonding the individual elements to insulating spacers. This positional deviation is the principal cause of variations in the optical coupling efficiency between the light-receiving and light-emitting elements.
To solve these problems, several couplingless, monolithic photocouplers have been proposed. Unfortunately, none of these monolithic photocouplers has satisfactory performance. In addition, many of these monolithic photocouplers are fabricated by stacking a light-receiving element, an insulating layer, and a light-emitting element by using a thin film formation method. However, by this method the insulating layer cannot have a large thickness, so it is impossible to impart the device a high dielectric withstand voltage.
Furthermore, in some monolithic photocouplers a light-receiving element and a light-emitting element are formed on both surfaces of a thick insulating layer such as glass. In the case of monolithic photocouplers of this type, however, a plurality of input and output electrodes connected to these elements are formed on both of the front and back sides of the substrate. This results in an extremely complicated mounting process.
Moreover, some monolithic photocouplers have a structure in which a mirror is formed on one surface of a thick glass substrate, light-receiving and light-emitting elements are arranged on the same surface, and light is reflected by the mirror to accomplish optical coupling. Unfortunately, in monolithic photocouplers of this sort the optical coupling efficiency is significantly low.